Class

This class lets us deal with the units in a ring.
isUnit tells whether an element is a unit.
The other operations let us canonically
write an element as a unit times another element.
Two elements a, b of a ring R are _associates_ if a=b*u for a unit u.
For an element a, we want to write it as a=b*u where b is an associate of a.
The map (a->b) is called
StandardAssociate by Gap,
unitCanonical by Axiom,
and canAssoc by DoCon.
The map (a->u) is called
canInv by DoCon and
unitNormal(x).unit by Axiom.